Feedback-assisted extension of the tokamak operating space to low safety factor

J.M. Hanson, J.M. Bialek, M. Baruzzo, T. Bolzonella, A.W. Hyatt, G.L. Jackson, J. King, R.J. La Haye, M.J. Lanctot, L. Marrelli, P. Martin, G.A. Navratil, M. Okabayashi, K.E.J. Olofsson, C. Paz-Soldan, P. Piovesan, C. Piron, L. Piron, D. Shiraki, E.J. StraitD. Terranova, F. Turco, A.D. Turnbull, P. Zanca

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Recent DIII-D and RFX-mod experiments have demonstrated stable tokamak operation at very low values of the edge safety factor q(a) near and below 2. The onset of n = 1 resistive wall mode (RWM) kink instabilities leads to a disruptive stability limit, encountered at q(a) = 2 (limiter plasmas) and q95= 2 (divertor plasmas). However, passively stable operation can be attained for q(a) and q95values as low as 2.2. RWM damping in the q(a) = 2 regime was measured using active MHD spectroscopy. Although consistent with theoretical predictions, the amplitude of the damped response does not increase significantly as the q(a) = 2 limit is approached, in contrast with damping measurements made approaching the pressure-driven RWM limit. Applying proportional gain magnetic feedback control of the n = 1 modes has resulted in stabilized operation with q95values reaching as low as 1.9 in DIII-D and q(a) reaching 1.55 in RFX-mod. In addition to being consistent with the q(a) = 2 external kink mode stability limit, the unstable modes have growth rates on the order of the characteristic wall eddy-current decay timescale in both devices, and a dominant m = 2 poloidal structure that is consistent with ideal MHD predictions. The experiments contribute to validating MHD stability theory and demonstrate that a key tokamak stability limit can be overcome with feedback. © 2014 AIP Publishing LLC.
Original languageEnglish
Article number072107
Pages (from-to)-
JournalPhysics of Plasmas
Issue number7
Publication statusPublished - 2014
Externally publishedYes


All Science Journal Classification (ASJC) codes

  • Condensed Matter Physics

Cite this

Hanson, J. M., Bialek, J. M., Baruzzo, M., Bolzonella, T., Hyatt, A. W., Jackson, G. L., ... Zanca, P. (2014). Feedback-assisted extension of the tokamak operating space to low safety factor. Physics of Plasmas, 21(7), -. [072107].